The .40 caliber blowgun measures 3/8", .50 caliber is 1/2" and the .62 caliber is 5/8".  These measurements are based on "inside" diameter of blowgun - wall to wall.  The blowgun is a very efficient weapon for hunting small game and is still used today. Blowguns come in many lengths and caliber's, each used for a different purpose. It's much harder to shoot a dart vertically, so when hunting Monkey's, Parrots, etc., an 8' to 10' blowgun is used to get the dart up 100' or more.  For small game a blowgun 3' to 6' is used.  For warfare the cigarette size to 30" is best suited. The small caliber blowguns with plastic cones and wires sold today are great for target use and are unbeatable for physical therapy, but for hunting they are not recommended unless a proper dart is constructed that makes a fairly good wound channel. Most blowguns used for hunting are .50 caliber or over.